1. Field of the Invention
The present invention relates to mode-switching transformers which are used to convert a voltage from the common mode to a differential mode and conversely. Such transformers are generally called “balun” (balanced-unbalanced) transformers.
2. Discussion of the Related Art
A mode-switching transformer is, for example, used in radio frequency transceiver chains, for example, of mobile phones. This type of application currently uses devices of balun type, the antenna side being most often associated with a single-ended device.
Two categories of mode-switching transformers are coupled-line or distributed baluns, and baluns with discrete components. Baluns with coupled lines are formed of pairs of coupled conductive tracks, the transformer's operating frequency being conditioned by the line length. Baluns with discrete components are formed of capacitive and inductive components forming LC cells.
The present invention more specifically relates to baluns with coupled lines. Among these, the present invention more specifically relates to a Marchand-type balun, that is, a symmetrical balun having its coupled lines calculated from a λ/4 length, where λ represents the wavelength corresponding to the central frequency of the desired passband of the balun.
FIG. 1 very schematically shows the function of a balun 1. On the common-mode side, the device includes a so-called common-mode port 2 (for example, intended to be connected to the end of an antenna). On the differential mode side, the balun includes two ports 3 and 4 symmetrical with respect to a reference M, generally the ground on the side of the equipment for processing the received signals. On the common mode side, the received signal is most often referenced to earth T. The grounds on the common mode and on the differential mode sides may (but not necessarily) exhibit potentials different from each other. For a perfectly symmetrical balun, a voltage V on the common mode side is converted into two voltages V/2 on the differential mode side.
FIG. 2 very schematically shows the equivalent electric diagram of a conventional Marchand type balun. Four conductive sections 5 to 8, each having a quarter wavelength (λ/4) are coupled two by two. Two first sections 5 and 6 are in series between common-mode port 2 and a port 9 generally open and left floating. The other two sections 7 and 8 are in series between the two differential mode ports 3 and 4 and have their midpoint 10 connected to ground M forming the reference on the differential side. The signals present on differential mode input-output terminals 3 and 4 are phase-shifted by 180° with respect to each other.
FIG. 3 very schematically shows in the form of blocks an example of an assembly using a balun 1 of the type to which the present invention applies. In this example, the common-mode port 2 is connected to the end of a radio frequency transceiver antenna 11 and the two differential ports are intended to be connected to circuits 12 of exploitation of the received signals and of preparation of the signals to be transmitted (APPLI). Between balun 1 and circuit(s) 12 is provided an impedance matching circuit 13 (ZMATCH), necessary in most applications.
A balun such as illustrated in FIG. 2 in which all the conductive sections have identical structures and same lengths provides a unity impedance ratio between the primary (common mode) and the secondary (differential mode).
According to the application, the circuits on the primary and secondary side may or may not have the same impedance (for example, 50 Ω). A non-unity impedance ratio may be required (for example, 50/100 between the antenna (50 Ω) and surface acoustic wave filters (100 Ω), 50/200, 25/50, 100/25, etc.).
Conventionally, to modify the impedance ratio of a balun with coupled lines, the spirals on one of the balun sides are connected in parallel. FIG. 4 shows the equivalent electric diagram of an example of such a balun with a non-unity impedance ratio. In FIG. 4, coupled lines 5 to 8 have been shown in the form of inductances since they most often are coupled spirals. On the primary side, the two spirals 5 and 6 are in series. On the secondary side, spirals 7 and 8 are (50 Ω) connected in parallel. Thus, for a voltage V present on the common-mode port 2 generating a current I in spirals 5 and 6 in series, a voltage V/2 is obtained across each spiral 7 and 8 but said spirals conduct a current 2I. Accordingly, the impedance ratio of such a balun here is 4 (for example, 200/50).
A problem to solve is the parallel connection of spirals 7 and 8. Indeed, baluns with coupled lines are most often made in the form of conductive tracks wound to gain space with respect to rectilinear tracks. The coupling between sections 5 and 7, respectively 6 and 8, is then obtained by forming these sections on the same level or in different conductive levels in interdigited and/or superposed fashion. Windings 5 and 6, respectively 7 and 8, are for example laterally spaced apart. In such a structure, the parallel connection of the two spirals requires additional vias and bridges. Further, for impedance ratios other than four or one quarter, the connections of multiple spirals in parallel make the forming of such baluns all the more complex.